During papermaking operations, an aqueous slurry of cellulosic fibers (furnish or pulp) is produced by subjecting wood to various mechanical and/or chemical processes with the aim of liberating and individualizing fibers from the wood stock. For some grades of pulp, the chemical treatment is geared toward removal of lignin from the fibers. The pulp is often subjected to additional chemical treatments such as bleaching where the fibers are further liberated and decolored to lighten the fibers for production of white paper or board product. Bleaching also tends to reduce fiber strength and viscosity, as the chemicals will attack the cellulose to some degree.
Once a pulp of the desired degree of liberation and whiteness if obtained, it is then diluted in water to make what is known as the furnish. The furnish is then sent to a headbox and released from a wide, slit-like opening known as a "slice" and the jet is deposited onto a continuously moving screen or "wire" (also sometimes referred to as a forming fabric) where the furnish is dewatered. As the papermaking furnish is dewatered, fibers and other particulates become trapped by the forming fabric, forming a paper web which is further processed into the end product. To maintain an acceptable rate of production on a commercial paper or paperboard machine, it is essential that the furnish permit sufficiently rapid drainage of the water from the forming mat of fibers.
The papermaker will often tailor the make-up of the furnish using various additives to produce a paper or board sheet with desired formation properties. Many of these additives are supplied as substantially water-insoluble particulate solids in a finely divided state. Exemplary solid particulate additives include clay, calcium carbonate, titanium dioxide, pigment dyes, rosin, and other materials. To achieve the desired effect, it is essential that these additives be efficiently retained in the web forming on the wire.
The furnish used by most papermakers is composed mainly of cellulosic fibers suspended in water such as groundwood pulp, chemithermomechanical pulp (CTMP), kraft pulp, or sulfite pulp. It is the nature of these pulp varieties to have, at the surfaces of fibers and other suspended particles, a net electrical charge. Charges on the surfaces of solids suspended in papermaking furnish arise for several reasons including (a) dissociation of ionizable groups such as carboxylic acids, sulfonic acids, amines, and hydrous oxides, (b) adsorption of ions from solution, and (c) substitution of atoms having differing valence into the lattices of inorganic crystals.
Charges at the surfaces of suspended particles attract ions of opposite charge (counter-ions), resulting in the formation of an ionic double-layer. Some of the counter-ions in the double layer are present immediately adjacent to the charged surface (Stern layer). Thermal energy causes some of the counter-ions to diffuse away from the surface so that they are present as a diffuse cloud of gradually decreasing density of net charge.
The net electrical charge of suspended particles within the furnish can have a profound effect on retention, drainage during web formation, and paper properties. For example, if a new batch of pulp contains a higher level of dissolved anionic polymers and colloidal anionic material (disco), then a decrease in the percentage of fine material retained during a single pass over the forming fabric (first-pass retention) is generally observed. An inherent difficulty, however, is that the colloidal charge of a papermaking furnish is subject to sudden changes. Sources of such disturbances include variations between different batches of pulp, transitions between different types of species of wood in pulping, the use of mechanical pulps of variable quality and variations in pulp washing efficiency, variations in the ratio between different types of pulp, in the amount of broke, in the concentrations of latex dispersants, carry-over of de-inking chemicals, in the level of hemicellulose and byproducts of oxidative bleaching, in micro-biological activity and in the quality of fresh water. Further disturbances in charge may result from changes in the levels of additives such as dyes, wet-strength additives, dry-strength resins, recycled size-press starch,-biocides, colloidal silica, bentonite, and other colloidally charged additives.
As a further source of colloidal charge disturbances, papermakers must deal with the consequences of intermittent interruptions of parts of the system. For example, when the web of paper breaks it must be rethreaded. Such interruptions have the tendency to move the system away from an optimum balance between the chemical additives having opposite colloidal charges, thereby making it more difficult for papermakers to maintain a uniform product.
Therefore, stability and control of the electrokinetic properties of the papermaking furnish is a necessary requirement for the efficient production of a uniform paper product.
Several approaches to monitoring and controlling the electrokinetic properties of a papermaking furnish have previously been attempted. These approaches include laboratory-based tests where a sample is withdrawn from the furnish or from the whitewater which drains from the furnish during web formation. One such lab-based test involves a process known as micro-electrophoresis, in which a microscope is used to observe the motion of a particle in the size range of about 0.2-500 microns as it migrates under the influence of an imposed electric field. The quotient of the particle velocity and the field strength defines the electrophoretic mobility. When the viscosity of the fluid and the conductivity are known, it is possible to calculate the "zeta potential" from this approach, which is the electrical potential at the plane of shear that divides the counter-ions, which tend to stay with the surface, from those ions which tend to move with aqueous fluid flowing past the surface.
Among the difficulties of micro-electrophoresis as applied to the study and control of papermaking operations is the requirement that the observed particles be much smaller than the diameter of the capillary cell in which the tests are conducted. This size requirement usually precludes direct microelectrophoretic analysis of the kind of cellulose fibers used in papermaking. Accordingly, the conventional approach is to screen the fibrous slurry and test the fine suspended solids remaining in the filtrate, or white-water. The capillaries used in micro-electrophoresis measurements are subject to contamination by the many polymeric and particulate ingredients in paper. Also, the focusing of the microscope tends to be too delicate for continuous operation in the industrial environment of a paper mill. An incorrectly focused micro-electrophoresis apparatus will give misleading data that is not representative of the actual zeta potential. A further problem with micro-electrophoresis is that the information obtained about the electrophoretic mobility, or zeta potential, is usually not linearly proportional with any process or additive which is under the direct control of the papermakers.
Another approach to monitoring the electrokinetic properties of papermaking furnish is to employ a device that measures "streaming current" of a sample of furnish. A typical streaming current device consists of a poly-tetrafluoroethylene (PTFE) plunger which is reciprocated within a PTFE cylinder containing the furnish sample. The reciprocating motion causes fluid to move rapidly in the annular space between the plunger and cylinder wall. The motion of the fluid induces a streaming current which is detected by connection of a suitable ammeter to electrodes positioned adjacent the annular space. An alternating signal, which is produced by the reciprocating fluid flow, is rectified and smoothed to produce the streaming current information. The accuracy of the streaming current method, however, depends strongly on levels of materials such as sodium sulfate and potassium chloride which are present in the samples being tested.
Yet another method for measuring the electrokinetic charge of papermaking furnish is known as "streaming potential". This method involves measuring the streaming potential resulting from liquid flow through a mat or plug of fibers and other particles collected on a screen. The streaming potential is determined from the change in electrical potential measured between electrodes located on either side of the fiber pad relative to a reference measurement or measurements at a relative pressure across the pad which is different from the first value. Usually, the reference data are obtained at a small or zero pressure difference across the mat or in the absence of a mat.
Streaming potential measurements can be used to gain information about the zeta potential at the surface of the actual fibers. The measurements are repeated often enough to give the papermaker a good idea of how the zeta potential varies as a function of time. However, the accuracy of the absolute magnitudes of the zeta potential is generally considered unreliable due to a perception that the results are dependent on the degree of compaction of the fiber mat. Additionally, streaming potential measurement devices do not provide the papermaker with information about colloidal charge.
Therefore, it is an object of the present invention to provide a method and apparatus for determining electrokinetic properties of a papermaking furnish in order to provide reliable formation-related data.
Another object of the present invention is to provide a method and apparatus which enables a papermaker to measure the colloidal charge of a papermaking furnish.
Yet another object of the invention is to provide a method and apparatus of the character described which promotes efficient use of fiber furnish additives such as highly charged cationic polymers.
A further object of the invention is to provide a method and apparatus of the character described which enables achievement of accurate diagnosis of the root causes of variations in production uniformity and product quality.
An additional object of the invention is to provide a method and apparatus of the character described which enables achievement of optimum performance of furnish additives, retention of fine materials, drainage rates, and uniformity of formation.
Still another object of the invention is to provide a method and apparatus for determining formation-related properties of a furnish to enable control of the web formation by necessary adjustment in the furnish composition.
Yet another object of the invention is to provide a method and apparatus which enables control of the formation properties of papermaking furnish by accurate determination of the electrokinetic charge of the furnish.